A fuel cell is a cell in which a fuel and an oxidant are supplied to two electrically-connected electrodes to electrochemically oxidize the fuel, thereby converting chemical energy directly to electrical energy. Accordingly, it is not limited by the Carnot cycle and shows high energy conversion efficiency. A fuel cell generally includes a stack of fuel cells, each having a membrane electrode assembly (MEA) as the basic structure which is composed of a pair of electrodes and an electrolyte membrane sandwiched therebetween.
Platinum and platinum alloy materials have been used as the electrode catalyst of a fuel electrode (anode electrode) and an oxidant electrode (cathode electrode) in a fuel cell. However, platinum in an amount that is required of an electrode catalyst using the latest technology, is still too expensive to realize a commercial mass production of fuel cells. Accordingly, there have been studies trying to reduce the amount of platinum contained in the fuel and oxidant electrodes of a fuel cell, by combining platinum with an inexpensive material.
In recent years, a fuel cell catalyst in which platinum-metal oxide composite particles are supported on an electroconductive support, has attracted attention as a fuel cell electrode catalyst. For example, an electrode catalyst for polymer electrolyte fuel cells is disclosed in Patent Literature 1, which is composed of composite particles made of platinum or a platinum-containing noble metal alloy and a metal oxide other than noble metals, and which is used for the air electrode of a polymer electrolyte fuel cell that includes a membrane electrode assembly composed of a solid polymer electrolyte membrane, which is permeable to protons, and fuel and air electrodes, each of which includes a catalyst layer containing an electrode catalyst.